Nanoscale plasmonic field-effect modulator

ABSTRACT

A plasmonic device having a transparent conducting oxide (TCO) waveguide and a tunable voltage applied across the TCO and a metal layer for modulating an input optical signal.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to U.S. Provisional PatentApplication No. 61/808,364 filed on Apr. 4, 2013, which is incorporatedherein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under FA9550-12-1-0024awarded by the Air Force. The government has certain rights in theinvention.

FIELD

The present invention relates to plasmonic waveguide modulators. Moreparticularly, it relates to nanoscale plasmonic field-effect modulators.

BACKGROUND

Optical modulators are used in a variety of applications, for example,in computer networks. One example of an optical modulator is a photonicoptical modulator where the optical signal propagates inside the bulk ofthe propagating material.

SUMMARY

A plasmonic field-effect modulator based on a transparent conductingoxide (TCO) active region is described. The TCO material is usedtogether with plasmonic waveguides to provide plasmonic waveguidemodulators with high dynamic range and low waveguide loss.

According to a first aspect, an optical modulator is described,comprising: a substrate; a metal layer on the substrate, the metal layerhaving a grooved channel; a dielectric layer on the metal layer and inthe grooved channel; a transparent conducting oxide (TCO) on thedielectric layer and in the grooved channel; and a tunable voltageapplied across the TCO and the metal layer.

The grooved channel comprises a first end of the grooved channel at afirst edge of the metal layer, and a second end of the grooved channelat a second edge of the metal layer.

An optical input signal is configured to be applied to the groovedchannel at the first end, and a modulated optical signal is configuredto be outputted at the second end.

The grooved channel forms a plasmonic waveguide such that an inputoptical signal applied to the first end of the grooved channel is outputfrom the second end of the grooved channel.

The grooved channel comprises a stub extending substantiallyperpendicularly to the grooved channel.

The grooved channel comprises a section having a first width and asection having a second width, wherein the width of the groove variesgradually from the first width to the second width, and from the secondwidth to the first width.

According to a second aspect, a plasmonic waveguide modulator isdescribed, comprising: a first accumulation layer formed by a firstmetal-dielectric interface; a second accumulation layer formed by asecond metal-dielectric interface, the first accumulation layer and thesecond accumulation layer being arranged next to and spaced from eachother, and such that a dielectric layer of the first interface and adielectric layer of the second interface face each other; transparentconducting oxide (TCO) between the first accumulation layer and thesecond accumulation layer; and a tunable voltage applied across the TCOand a metal layer of the first interface and the second interface.

The TCO is configured to propagate an input optical signal by confiningthe input optical signal in the grooved channel.

The grooved channel comprises a first section having a first width and asecond section having a second width.

The first accumulation layer comprises a first stub extending in adirection away from the second accumulation layer.

The first stub forms a cavity resonator along a plasmonic opticalpropagation path formed by the first interface, the second interface andthe TCO.

According to a third aspect, a method for modulating an optical signalis described, the method comprising: receiving, with a plasmonicwaveguide, an input optical signal, the plasmonic waveguide comprising:a metal layer on a substrate, the metal layer having a grooved channel,a dielectric layer on the metal layer and in the grooved channel, and atransparent conducting oxide (TCO) on the dielectric layer and in thegrooved channel; applying a tunable voltage across the TCO and the metallayer; and turning on and/or turning off the tunable voltage, thuscutting off and/or transmitting the optical signal in the groovedchannel.

Transmitting the optical signal comprises propagating the optical signalin a plasmonic gap mode, such that the propagating optical signal isconfined within the grooved channel.

The turning on and/or turning off the tunable voltages comprisesiteratively turning on and/or turning off the tunable voltage at a setfrequency.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention, and many of theattendant features and aspects thereof, will become more readilyapparent as the invention becomes better understood by reference to thefollowing detailed description when considered in conjunction with theaccompanying drawings in which like reference symbols indicate likecomponents.

FIG. 1 is a perspective view of a portion of a nanoscale plasmonicwaveguide modulator according to an embodiment.

FIG. 2 is a cross-sectional view of the nanoscale plasmonic waveguidemodulator according to an embodiment.

FIG. 3 is a graphical representation of carrier densities in the variouslayers of material of the nanoscale plasmonic waveguide modulator.

FIG. 4 is a diagram showing a plasmonic mode field distribution for thenanoscale plasmonic waveguide modulator according to an embodiment.

FIGS. 5A-5B are graphical representations of transmission or measuredloss of an optical input signal for various bias voltages.

FIG. 6 is a perspective view of a portion of a nanoscale plasmonicwaveguide modulator according to an embodiment.

FIG. 7 is a cross-sectional view of the nanoscale plasmonic waveguidemodulator according to an embodiment.

FIG. 8 is a diagram showing a plasmonic mode field distribution for thenanoscale plasmonic waveguide modulator according to an embodiment.

FIG. 9 is a graphical representation of plasmonic transmission spectraat various wavelengths for various stub sizes according to anembodiment.

FIG. 10 is a cross-sectional view of the nanoscale plasmonic waveguidemodulator according to an embodiment.

FIG. 11 is a graphical representation of transmission in the nanoscaleplasmonic waveguide modulator according to an embodiment.

FIG. 12 is a perspective view of a portion of a nanoscale plasmonicwaveguide modulator according to an embodiment.

FIG. 13 is a cross-sectional view of the nanoscale plasmonic waveguidemodulator according to an embodiment.

DETAILED DESCRIPTION

The present invention will now be described more fully with reference tothe accompanying drawings, in which example embodiments thereof areshown. While the described embodiments of the invention may be modifiedin various ways, the described embodiments are presented as examples inthe drawings and in the detailed description below. The intention of thedisclosure, however, is not to limit the invention to the particularembodiments described. To the contrary, the invention is intended tocover all modifications, equivalents, and alternatives falling withinthe scope of the invention as defined by the appended claims. Moreover,detailed descriptions related to well-known functions or configurationshave been omitted in order not to unnecessarily obscure the subjectmatter of the present invention.

The sizes of the layers and regions in the drawings may be exaggeratedfor convenience of explanation. Like reference numerals refer to likeelements throughout. It will be understood that when a layer, region, orcomponent is referred to as being “on”, “formed on”, “over”, or “formedover”, another layer, region, or component, it can be directly orindirectly on or formed on the other layer, region, or component. Thatis, for example, intervening layers, regions, or components may bepresent.

Active control of light at the nanoscale level with plasmonic waveguideshas a variety of applications in photonic chip integration. Transparentconducting oxides (TCOs) have high transmission at optical wavelengthsand therefore, are desirable for integration into active plasmonicstructures. TCO can be used together with plasmonic waveguides toprovide plasmonic waveguide modulators with high dynamic range (˜2dB/μm) and low waveguide loss (˜0.4 dB/μm). Carrier concentrations ofTCOs can be significantly altered via application of electrical bias,similar to the field-effect of MOS-devices where an accumulation layeris formed due to the applied electric field, subsequently inducing achange in the local index of refraction. When the TCO is arranged in aTCO/dielectric/metal configuration, an electrical bias can be applied tothe TCO and the metal to achieve a plasmonic waveguide modulator as aresult of an accumulation layer formed due to the field-effect from thebias, similar to the field-effect in a metal oxide semiconductorfield-effect transistor (MOSFET). Because of the high modal confinementachievable in plasmonic waveguides, TCO based waveguides can cause largechanges to the effective index of plasmonic modes through thefield-effect mechanism, thus offering high speed, power efficientmodulators with low insertion losses.

According to an embodiment of the invention, a plasmonic waveguide iscreated by forming a grooved channel (a gap) in a metal layer, andfilling the grooved channel with TCO. FIG. 1 illustrates a perspectiveview of a thin (e.g., ˜200 nm) metal layer 102 formed on a semiconductorsubstrate 100. A gap is formed in the metal layer 102, thus creating agrooved channel 112 that extends from a first edge of the metal layer102 to a second edge of the metal layer 102. In some embodiments, thegap may be, but is not necessarily limited to, approximately 200-300 nm.However, in some embodiments, the gap may be as narrow as 10 nm, or aswide as 1,000 nm. In some embodiments, the grooved channel 112 is asubstantially straight grooved channel 112, extending directly acrossfrom the first edge of the metal layer 102 to an opposite edge of themetal layer. In other embodiments, the grooved channel 112 is curved orbent as shown in FIG. 1. A curved or bent grooved channel 112 may reducethe amount of scattering of the input optical signal by across-polarization detection scheme, thus improving signal-to-noiseratio detection in the output signal. For purposes of this disclosure,the embodiments will be described with respect to a grooved channel 112having a 90 degree bend by way of example, and is not intended to belimiting. A person skilled in the art will realize that the groovedchannel 112 may take different paths, straight or curved, based onfactors such as shape, size, and/or application of the plasmonicwaveguide 101.

In some embodiments, the grooved channel 112 is not necessarily limitedto square grooves having a bottom wall and two sidewalls, as shown inthe embodiment of FIG. 1. Instead, the grooved channel may be, forexample, a V-shaped grooved channel.

FIG. 2 is a cross-sectional view of the plasmonic waveguide 101 of FIG.1, according to an embodiment of the invention. The gap created by themetal layer 102 exposes a portion of the substrate 100 below the metallayer 102. A dielectric layer 108 is formed on the metal layer 102, andin the grooved channel 112, thus covering the exposed portion of thesubstrate 100 and the sidewalls 114 of the metal layer 102 formed as aresult of the gap. Therefore, the plasmonic waveguide 101 is arranged ina semiconductor/metal/dielectric/TCO layer configuration. Morespecifically, in one embodiment, it is a silicon dioxide/gold/aluminumdioxide/indium tin oxide (ITO) layer configuration. The section insidethe grooved channel 112 is arranged in a semiconductor/dielectric/TCOconfiguration because the metal layer 102 is removed from the groovedchannel 112. In some embodiments, the dielectric layer 108 is about 5-10nm, and the thickness of the dielectric layer 108 is substantially thesame along the sidewalls 114, the bottom of the grooved channel 112 andon the metal layer 102. However, in some embodiments, the dielectriclayer 108 may even be as thin as 1 nm, or as thick as 200 nm. Thedielectric layer 108 serves as an insulator between the metal layer 102and the TCO 110 so that current does not pass through the dielectriclayer 108 (insulator) and a voltage difference is established betweenthe metal layer 102 and the TCO 110. By way of example, the metal layer102 may be gold, silver, or aluminum, or a combination thereof, but notnecessarily limited to these metals. By way of example, the dielectriclayer 108 can be aluminum dioxide, silicon dioxide, or other insulators.

In some embodiments, the gap created by the metal layer 102 may notnecessarily expose a portion of the substrate 100. Instead, the groovedchannel in the metal layer may have a metal bottom wall. That is, agroove is formed in the metal layer 102, but the groove does notcompletely reach the substrate layer. Thus, according to thisembodiment, the dielectric layer within the grooved channel is alsoformed on the metal layer, and the TCO is formed on the dielectriclayer, similar to the other regions of the plasmonic waveguide.

According to an embodiment of the invention, a TCO layer 110 is formedon the dielectric layer 108, completely covering the grooved channel112. Thus, the TCO layer 110 is about 300 nm thick above the dielectriclayer 108, and the grooved channel 112 portion is completely coveredwith the TCO layer 110. Therefore, the TCO layer 110 in the region overthe grooved channel 112 is thicker than the TCO layer 110 in the regionover the metal layer 102. However, the TCO contacts all portions of thedielectric layer 108, including the sidewall 114 portions of the groovedchannel 112. The thickness of the TCO layer 110 can be made thicker orthinner according to the application for which the plasmonic waveguideis being used. For example, in some embodiments, the TCO layer 110 maybe thicker to increase confinement of the optical signal, or vice versa.In some embodiments, the TCO layer 110 may be as thin as 50 nm, or asthick as 1,000 nm. By way of example, the TCO 110 can be indium tinoxide (ITO), or gallium zinc oxide (Ga:ZnO), aluminium zinc oxide(Al:ZnO) or other transparent conductive oxides. While the embodimentshown in FIG. 2 shows a dip or a depressed portion of the TCO layer 110over the grooved channel 112, the dip is not necessary. Instead, the dipmay be formed merely due to a sputtering from fabrication process of theplasmonic waveguide 101.

FIG. 3 illustrates a field distribution in the plasmonic waveguide 101of FIGS. 1-2, according to the described embodiment. The fielddistribution shows that the optical signal is confined within thegrooved channel 112, with minimal scattering in the surrounding areas.An input optical signal is confined within the grooved channel 112 andpropagates in plasmonic mode from an input 104 of the plasmonicwaveguide 101 to an output 106 of the plasmonic waveguide 101 (see FIG.1). That is, the input optical signal coupled to the input 104 via, forexample, an optical coupling device (e.g., optical fiber, opticalantenna, etc.), and the surface plasmon polariton (SPP) of the inputoptical signal propagates inside the grooved channel 112. Generally, inplasmonic mode, SPP travels along the metal/dielectric interface.However, when two metal/dielectric interfaces are positioned facing eachother with a gap therebetween, the two interfaces create the groovedchannel 112 described herein, and operate in a “gap” plasmonic modewhere the input optical signal propagates inside the grooved channel 112instead of at the interface of the metal/dielectric layer. Accordingly,in some embodiments, the optical signal can propagate for a longerdistance in the gap plasmonic mode with two interfaces when comparedwith the general plasmonic mode having one interface.

According to an embodiment of the invention, an electrical bias voltageis applied by a voltage source 116 across the TCO layer 110 and themetal layer 102. Thus, when the bias is applied, an accumulation layeris formed at the metal/dielectric interface (e.g., Al₂O₃/ITO interface)that subsequently changes the properties of the propagation mode in thegrooved channel 112. That is, when the bias is applied across the TCOlayer 110 and the metal layer 102, the carrier concentration of the TCOis changed such that the input optical signal is cutoff, or nearlycutoff, and the total transmission is reduced such that the opticalsignal is substantially non-detectable at the output of the plasmonicdevice 101. This state corresponds to an OFF state if the plasmonicwaveguide device is used, for example, as a switch. In some embodiments,the input optical signal can be cutoff by about 30 dB (i.e., anextinction ratio of about 30 dB or more). Therefore, the plasmonicwaveguide can behave as an optical switch by merely turning ON/OFF theelectrical bias voltage, and rapidly cycling or modulating the biasvoltage establishes a plasmonic waveguide modulator.

FIG. 4 is a graphical representation of carrier densities in the TCOlayer 110, the dielectric layer 108, and the metal layer 102, of theplasmonic waveguide modulator according to an embodiment of theinvention. In this embodiment, a positive bias is applied to the TCOlayer 110 and a negative bias is applied to the metal layer 102. Asrecognized by a person skilled in the art, the carrier density of themetal layer 102 (e.g., gold) is relatively high, and the carrier densityof the dielectric layer 108 is relatively low. While the carrier densityof the TCO layer 110 near the center (i.e., away from the adjacentdielectric layers 108) is shown to be approximately 10²¹ cm⁻³ 117, thecarrier densities of the TCO layer 110 adjacent the dielectric layers108 (i.e., at the TCO/dielectric interface) is much higher as seen at118. That is, the carrier density of the accumulation layer formed as aresult of the electrical bias, has carrier concentrations higher thanthe remaining areas of the TCO layer. Therefore, the electrical biasvoltage can be increased or decreased to vary the carrier densities atthe interface, thus changing the amount of the input optical signal topropagate in the grooved channel 112.

FIG. 5A is a graphical representation of the input optical signaltransmission through the grooved channel 112 of the plasmonic waveguide101 as a function of different bias voltages. While this graphicalrepresentation shows these effects for voltages in the range of 0-2.5 V,other voltage ranges such as, for example, 0-5 V are possible. Forexample, if the applied bias voltage is higher than a breakdown voltageof the dielectric material comprising the dielectric layer 108, then thedielectric material may become damaged. Therefore, in some embodiments,the maximum bias voltage is determined based on the thickness of thedielectric layer 108. By way of example, a 3 V bias voltage may appliedfor a dielectric layer 108 (oxide layer) of 5 nm. However, the voltagemay be slightly higher or lower depending on the type of dielectriclayer. According to the embodiment, the transmission strength decreasesas the bias voltages increases. In some embodiments, the size of thewaveguide affects transmission of the optical signal through theplasmonic waveguide 101. For example, longer lengths of the groovedchannel 112 result in greater loss of the propagating optical signal.Accordingly, FIG. 5A shows the transmission results for four exemplarylengths of the waveguide. For example, transmission for a plasmonicwaveguide with the longest length, 35.28 μm (out of the four exemplarywaveguide lengths), has a lower transmission at a given bias voltagewhen compared with the transmission for a plasmonic waveguide withshorter lengths. For example, when a voltage of approximately 1.8V isapplied to the plasmonic waveguide, the transmission for the longestwaveguide (35.28 μm) decreases significantly, while in the case of theshorter waveguide (10.28 μm), the transmission level is relatively high.That is, with a bias of about 1.8 V, the optical signal in the 35.28 μmplasmonic waveguide is nearly cutoff (−30 dB), whereas the opticalsignal in the 10.28 μm plasmonic waveguide is still transmitted (−2 dB).

FIG. 5B shows the optical signal loss for every micrometer of theplasmonic propagation in the plasmonic waveguide at different biasvoltages. In one embodiment, the loss is relatively constant (0.5-0.7dB/μm) for bias voltages of about 0 V to about 1.5 V, but the lossincreases significantly more for bias voltages greater than about 1.5 V.For example, the loss when a 2 V bias voltage is applied is about 2dB/μm. Therefore, the bias voltage can be cycled between a relativelysmaller voltage (a voltage that allows more transmission) and arelatively larger voltage (a voltage that causes more loss) to create aplasmonic waveguide based optical modulator, according to the variousembodiments of the invention. For example, a plasmonic waveguide can bea 50 GHz optical modulator when the bias voltage is cycled at afrequency of 50 GHz. The bias voltages and the optical signaltransmission and loss shown in FIGS. 5A-5B are provided as exemplaryembodiments, and are not intended to necessarily be limited to theseparameters.

One quality of a desirable optical switch in some applications is a highextinction ratio (or modulation strength) to have a distinguishing ONstate and an OFF state. Another quality may be an optical switch thathas minimum scattering of the optical signal, which reduces insertionlosses. To further enhance the modulation strength and reduce theoperation voltage to minimize power consumption for modulation, a cavityresonator can be established in the grooved channel 112 of the plasmonicwaveguide by forming a further indentation in the gap of the groovedchannel. This indentation is be referred to herein as a stub. FIG. 6shows a perspective view of a plasmonic waveguide with a stub 120 formednear the center of the grooved channel 112.

FIG. 7 is a cross-sectional top-view of the plasmonic waveguidemodulator according to an embodiment of the invention. According to theembodiment, the stub 120 is formed along the inside of the groovedchannel 112, and extends into one side of the metal layer 102. That is,the stub 120 is a further gap in the metal layer 102, and is coveredwith the dielectric layer 108, and filled with TCO, similar to the restof the grooved channel 112. A localized Fabry-Perot resonance is formedin and around the region of the cavity as shown in the magnetic fielddiagram of FIG. 8. The formation of the stub allows the light toresonate near the region of the stub 120, thus improving confinement ofthe optical signal. Since it is desirable in some embodiments to reducescattering of the optical signal near the input 104 and output 106, thestub 120 may be placed near the center of the grooved channel 112 (awayfrom the input 104 and the output 106 ends of the grooved channel 112).

In some embodiments, the plasmonic waveguide is tunable to set resonantwavelengths. The carrier concentration at the accumulation layer regioncan be changed by varying the electrical bias such that the waveguidesubstantially cuts off the optical signal at different resonantwavelengths. According to an exemplary embodiment, for a 5 nmaccumulation layer with a carrier concentration of 1×10¹⁹ cm⁻³, theresonant wavelength of the plasmonic waveguide is 1630 nm, while for acarrier concentration 2.5×10²⁰ cm⁻³, the resonant wavelength is 1580 nm.Therefore, by varying the bias voltage to vary the carrierconcentration, the plasrnonic waveguide can be tuned according to adesired resonant wavelength based on the application of the plasmonicwaveguide device.

In the embodiment shown in FIG. 7, the stub 120 has a length L_(c) of400 nm and a width W_(c) of 70 nm for a plasmonic waveguide having agrooved channel width W (i.e., gap width) of 300 nm. However, the stub120 can have other dimensions in order to tune the plasmonic waveguideto have a desired resonant wavelength. Accordingly, FIG. 9 shows anexemplary comparison of the various resonant wavelengths for varyingstub 120 widths W_(c), ranging from 70 nm to 250 nm. Smaller widthsprovide higher resonant wavelengths, while larger widths W_(c) providelower resonant wavelengths. However, the dimensions are not limitedthereto, and may even range from as low as 10 nm to as high as 500 nm.In some embodiments, the stub can have different shapes, as long as theresonant cavity is created in the grooved channel, and is notnecessarily limited to the shape shown herein. For example, the stub maybe trapezoidal or ellipsoidal.

According to another embodiment, the plasmonic waveguide modulator canhave two stubs (a first stub 122 and a second stub 124) formed along theinside of the grooved channel 112, as shown in FIG. 10. The double stubembodiment improves confinement of the optical signal, which increasesthe modulation strength. For example, according to the exemplaryembodiment of FIG. 10 where each of the two stubs have a length L_(c) of400 nm and a width W_(c) of 70 nm, and a distance of 180 nm from thecenter of the first stub 122 to the center of the second stub 124, themodulation strength is about 30.4 dB for a wavelength of λ=1630 nm asshown in FIG. 11, thus greatly improving the resonance based modulationefficiency. Accordingly, an optical modulator having a bandwidth ofgreater than 100 GHz and a switching energy of less than 5 fJ/bit can beachieved in a micro-length plasmonic modulator. While the modulatorhaving one or two stubs is described, modulators with more than twostubs can also be created. The spacing between the stubs is anotherfactor that can be considered when tuning the modulator to the desired,optimal resonant wavelength.

FIG. 12 is a perspective view of the plasmonic waveguide modulatoraccording to another embodiment of the invention. According to thisembodiment, instead of providing a stub 120 along the grooved channel112 as in the embodiment of FIG. 6, a portion of the grooved channel 112has an adiabatically tapered region 126, which increases the modulationstrength of the plasmonic waveguide modulator.

FIG. 13 is a cross-sectional top-view of the plasmonic waveguide of FIG.12. The tapered region 126 is shown as a section of the grooved channel112 that has a narrower width W relative to the remaining portion of thegrooved channel 112. The region between the wider portion of the groovedchannel and the narrower portion of the grooved channel is tapered. Insome embodiments, the wider region of the grooved channel isapproximately 300 nm, as described in the other embodiments, and widthof the tapered region 126 is approximately 120 nm. The tapered region126 is approximately 5 μm long. According to the embodiment, a longertapered region results in a stronger modulation strength, and a higherpropagation loss. A shorter tapered region results in weaker modulationstrength, and less propagation loss. Additionally, a wider channel ofthe tapered region 126 provides less confinement of the optical signal,and thus results in weaker modulation strength. A narrower channel ofthe tapered region 126 provides greater confinement of the opticalsignal, and thus results in greater modulation strength.

As described according to the various embodiment of the invention, aplasmonic waveguide can modulate the input optical signal by switchingON/OFF the waveguide by modulating the bias voltage. By way of example,the bias voltage can be provided by an off-the-shelf voltage source orvoltage modulator.

Accordingly, the plasmonic waveguide modulator can be made ultra-compactwhen compared in size to, for example, a photonic waveguide modulator.While the optical signal propagates inside the bulk of the material in aphotonic waveguide, in the case of a plasmonic waveguide, the opticalwaves propagate as SPP along the surface, or at the interface of themetal/dielectric layers. Thus, the size of the plasmonic waveguide canbe reduced since the optical waves are not contained inside the bulk ofa material. Rather, they are confined to an interface between twomaterials. For example, the plasmonic waveguide modulator may be assmall as 300 nm or less. Therefore, the plasmonic waveguide modulator,described according to the embodiments, take up less space and consumeless power, yet still provide faster modulation of about 100 GHz or morebased on the embodiments of the described invention.

In some embodiments, the plasmonic waveguide modulator can be fabricatedwith electron beam lithography on silica, with about 200 nm metal layersuch as gold layer with a waveguide gap width W of about 300 nm. In someembodiments, the metal layer 102 may be as thin as 50 nm, or as thick as1,000 nm. A layer (5-10 nm) of aluminum oxide is deposited for isolationand a TCO such as ITO is sputtered on top as an active plasmonicmaterial, with the gold and ITO layers serving as the bias voltagecontacts (see FIG. 2) for the modulator. A 2.5 dB extinction ratiomodulation can be achieved with an applied bias of, for example, 5 V fora 100 nm wide, 1 μm long waveguide structure where the unmodulated,unbiased ITO carrier concentration is n 1×10¹⁹ cm⁻³. However, theparameters described above provide one example embodiment and a methodfor manufacturing the same, and is not intended to limit otherparameters from the scope of the invention. Moreover, modifying one ormore of the described parameters allows a person skilled in the art toprecisely tune a desired plasmonic waveguide modulator that is optimalfor the intended application.

Table 1 shows an example comparison between the plasmonic waveguidemodulator according to the described embodiments (rows 8-9) and otherdevices (rows 1-7). See, for example, A. Joushaghani, et al., Appl.Phys. Lett. 102, 061101 (2013); V. J. Sorger et al., Nanophotonics 1, 17(2012); J. A. Dionne et al., Nano Lett. 9, 897 (2009); S. Randhawa etal., Opt. Express 20, 2354 (2012); S. Papaioannou et al., Sci. Rep. 2,652 (2012); M. R. Watts et al., Opt. Express 19, 21989 (2011); J. Liu etal., Nat. Photonics 2, 433 (2008), the contents of which areincorporated herein by reference in their entirety.

TABLE 1 Extinction Extinction Insertion Modulation Length RatioRatio/Length Loss Voltage Bandwidth Device Type Mechanism (μm) (dB)(dB/μm) (dB) (V) (nm) 1 Hybrid SPP-VO2 waveguide VO2 phase 7 16.4 2.3 60.4 >100 transistion 2 Hybrid SPP-ITO waveguide ITO plasma 5 5 1 14.4 >100 dispersion 3 Field-effect MIM waveguide Si plasma 2.2 4.6 — 40.8 >100 dispersion 4 Dielectric loaded SPP ring Electro-optic 35 0.70.02 — 32 10 resonator polymer 5 DLSPP Mach-Zehnder Thermo-optic 60 140.23 11 0.33 — interferometer polymer 6 Si double microdisk Si plasma12.5 16 1.3 4 0.6 0.4 dispersion 7 GeSi electroabsorption Franz-Keldysh50 10 0.5 6 0.05 1.4 effect 8 Straight plasmon waveguide ITOfield-effect 10 >30 >2 4 2 >100 9 Stub resonant waveguide ITOfield-effect <1 >30 >30 <0.7

Although the terms “first”, “second”, etc. may be used herein todescribe various components, these components should not be limited bythese terms. These descriptors are used only to distinguish onecomponent from another. The terminology in this application is used tomore clearly describe the presented embodiments and is not intended tolimit the scope of the present invention.

As used herein, the singular forms “a”, “an”, and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising”, as used herein, specify the presence of the statedfeatures or components, but do not preclude the presence or addition ofone or more other features or components. “/”, as used herein may beinterpreted as “and”, or may be interpreted as “or” depending on thesituation.

It will be recognized by those skilled in the art that variousmodifications may be made to the illustrated and other embodiments ofthe invention described above, without departing from the broadinventive step thereof. Therefore, the invention is not limited to theparticular embodiments or arrangements disclosed, but is rather intendedto cover any changes, adaptations or modifications which are within thescope and spirit of the invention as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. An optical modulator, comprising: a substrate; ametal layer on the substrate, the metal layer having a grooved channel;a dielectric layer on the metal layer and in the grooved channel; and atransparent conducting oxide (TCO) on the dielectric layer and in thegrooved channel, the grooved channel being a plasmonic waveguide,wherein the plasmonic waveguide is configured to form an accumulationlayer in response to a voltage applied across the TCO and the metallayer such that the accumulation layer cuts off a fundamental mode of anoptical signal in the plasmonic waveguide and such that the plasmonicwaveguide exhibits an extinction ratio of at least about 3.0 dB/μm. 2.The modulator of claim 1, wherein the grooved channel comprises a firstend of the grooved channel at a first edge of the metal layer, and asecond end of the grooved channel at a second edge of the metal layer.3. The modulator of claim 2, wherein the optical input signal isconfigured to be applied to the grooved channel at the first end, and amodulated optical signal is configured to be outputted at the secondend.
 4. The modulator of claim 2, wherein the optical signal applied tothe first end of the grooved channel is output from the second end ofthe grooved channel.
 5. The modulator of claim 1, wherein an entireportion of the grooved channel is covered with the TCO.
 6. The modulatorof claim 1, wherein the grooved channel comprises a stub extendingsubstantially perpendicularly to the grooved channel.
 7. The modulatorof claim 6, wherein the stub is formed near a center section of a lengthof the grooved channel.
 8. The modulator of claim 6, wherein the stubforms a cavity resonator in the grooved channel.
 9. The modulator ofclaim 6, wherein the stub comprises two or more stubs, each of the stubsbeing spaced from one another.
 10. The modulator of claim 1, wherein thegrooved channel comprises a section having a first width and a sectionhaving a second width, wherein the width of the groove varies graduallyfrom the first width to the second width, and from the second width tothe first width.
 11. The modulator of claim 10, wherein the first widthis wider than the second width.
 12. The modulator of claim 1, whereinthe grooved channel is V-shaped.
 13. The modulator of claim 1, whereinthe TCO is selected from the group consisting of: indium tin oxide(ITO), gallium zinc oxide (Ga:ZnO), and aluminum zinc oxide (Al:ZnO).14. The modulator of claim 1, wherein the metal layer is a gold layer ora silver layer.
 15. The modulator of claim 1, wherein the groovedchannel in the metal layer extends to the substrate.
 16. A plasmonicwaveguide modulator, comprising: a first accumulation layer formed by afirst metal-dielectric interface; a second accumulation layer formed bya second metal-dielectric interface, the first accumulation layer andthe second accumulation layer being arranged next to and spaced fromeach other, and such that a dielectric layer of the first interface anda dielectric layer of the second interface face each other; transparentconducting oxide (TCO) between the first accumulation layer and thesecond accumulation layer; and a tunable voltage applied across the TCOand a metal layer of the first interface and the second interface,wherein a grooved channel formed by the first interface, the secondinterface, and the TCO is a plasmonic waveguide, and wherein the firstaccumulation layer and the second accumulation layer cuts off afundamental mode of an optical signal in the plasmonic waveguide andwherein the plasmonic waveguide exhibits an extinction ratio of at leastabout 3.0 dB/μm.
 17. The modulator of claim 16, wherein the TCO isconfigured to propagate an input optical signal by confining the inputoptical signal in the grooved channel.
 18. The modulator of claim 16,wherein the grooved channel comprises a first section having a firstwidth and a second section having a second width.
 19. The modulator ofclaim 18, wherein the first width is formed by the first accumulationlayer being spaced from the second accumulation layer by a firstdistance, and wherein the second width is formed by the firstaccumulation layer being spaced from the second accumulation layer by asecond distance.
 20. The modulator of claim 19, wherein a majoritysection of the grooved channel has a first width, and a minority sectionof the grooved channel near a center of the grooved channel has a secondwidth.
 21. The modulator of claim 20, wherein the second section of thegrooved channel is located at a substantially center section of a lengthof the grooved channel.
 22. The modulator of claim 16, wherein the firstaccumulation layer comprises a first stub extending in a direction awayfrom the second accumulation layer.
 23. The modulator of claim 22,wherein the first stub forms a cavity resonator along a plasmonicoptical propagation path formed by the first interface, the secondinterface and the TCO.
 24. The modulator of claim 22, wherein the firstaccumulation layer further comprises a second stub spaced from the firststub, and extending in a direction away from the second accumulationlayer.
 25. A method for modulating an optical signal, the methodcomprising: receiving, with a plasmonic waveguide, an input opticalsignal, the plasmonic waveguide comprising: a metal layer on asubstrate, the metal layer having a grooved channel, a dielectric layeron the metal layer and in the grooved channel, and a transparentconducting oxide (TCO) on the dielectric layer and in the groovedchannel; applying a tunable voltage across the TCO and the metal layer;and cutting off a fundamental mode of the optical signal in response toturning on the tunable voltage such that the plasmonic waveguideexhibits an extinction ratio of at least about 3.0 dB/μm.
 26. The methodof claim 25, wherein the transmitting of the optical signal comprisespropagating the optical signal in a plasmonic gap mode, such that thepropagating optical signal is confined within the grooved channel. 27.The method of claim 25, wherein the turning on and/or turning off thetunable voltages comprises iteratively turning on and/or turning off thetunable voltage at a set frequency.
 28. The method of claim 25, whereinthe turning on the tunable voltage forms an accumulation layer at aninterface of the metal layer and the dielectric layer.